As an exercise in my Scheme (R6RS) learning journey, I have implemented a dataframe record type and procedures to work with the dataframe record type. Dataframes are column-oriented, tabular data structures useful for data analysis found in several languages including R, Python, Julia, and Go. In this post, I will introduce the dataframe record type and basic procedures for working with dataframes. In subsequent posts, I will describe other dataframe procedures, e.g., filter, sort, aggregate, etc.

Series record type

A dataframe is based on the series record type. A series record is a list where every element is the same type (one of bool, chr, str, sym, num, or other). The series record type includes the series name, source list (src), converted list (lst), list type, and list length. Only name and src are required to create a series; lst, type, and length are derived from src. Failed type conversions produce 'na values.

define-record-type creates a predicate, series?, constructor procedure, make-series, and accessor procedures for each field: series-name, series-lst, series-type, and series-length. [series-src is also created, but not exported to the dataframe namespace.] make-series* is macro to provide alternative syntax for making a series.

(define-record-type series
  (fields name src lst type length)
  (protocol
    (lambda (new)
      (lambda (name src)
        (check-series name src "(make-series name src)")
        (let* ([type (guess-type src 1000)]
               [lst (convert-type src type)])
          (new name src lst type (length lst)))))))

> (define s1 (make-series 'a '(1 2 3)))
> (define s2 (make-series* (a 1 2 3)))

> (series-equal? s1 s2)
#t

> (series? s1)
#t

> (series-name s1)
a

> (series-lst s1)
(1 2 3)

> (series-length s1)
3

Dataframe record type

The dataframe record type is based on a list of series (slist). The names and dim are derived from the slist. 

(define-record-type dataframe
  (fields slist names dim)
  (protocol
    (lambda (new)
      (lambda (slist)
        (check-slist slist "(make-dataframe slist)")
        (let* ([names (map series-name slist)]
               [rows (series-length (car slist))]
               [cols (length names)])
          (new slist names (cons rows cols)))))))

A key component of the record definition is check-slist, which confirms that all elements of slist are series with the same length and unique names. define-record-type creates a predicate, dataframe?, constructor procedure, make-dataframe, and accessor procedures for each field: dataframe-slist, dataframe-names, and dataframe-dim. make-df* is macro to provide alternative syntax for making a dataframe.

> (define df (make-df* (a 1 2 3) (b 4 5 6)))

> df
#[#{dataframe f4aik6efdmw9tjrhx8ell3b2e-58} (#[#{series f4aik6efdmw9tjrhx8ell3b2e-59} a (1 2 3) (1 2 3) num 3] #[#{series f4aik6efdmw9tjrhx8ell3b2e-59} b (4 5 6) (4 5 6) num 3]) (a b) (3 . 2)]

> (dataframe? df)
#t

> (dataframe-slist df)  
(#[#{series f4aik6efdmw9tjrhx8ell3b2e-59} a (1 2 3) (1 2 3) num 3]
  #[#{series f4aik6efdmw9tjrhx8ell3b2e-59} b (4 5 6) (4 5 6) num 3])

> (dataframe-names df)
(a b)

> (dataframe-dim df)
(3 . 2)                  ; (rows . columns)

> (make-df* (a 1 2 3) (a 4 5 6))
Exception in (make-dataframe slist): names not unique

> (dataframe-display df)

 dim: 3 rows x 2 cols
       a       b 
   <num>   <num> 
      1.      4. 
      2.      5. 
      3.      6.

Head and tail

In R, I frequently use head to preview the first few rows of a dataframe and, less frequently, use tail to view the last few rows. Scheme provides list-head and list-tail with similar functionality. However, tail in R returns the last n rows of the dataframe whereas list-tail in Scheme returns the rest of the list starting at a given index. My first instinct was to write dataframe-tail to use the R behavior, but eventually decided that dataframe-tail should follow the behavior established by list-tail. I was trying to think in terms of the principle of least surprise, but the degree of surprise depends on the potential users. Am I targeting R or Scheme programmers? The most realistic scenario is that future me is the only potential user and I want that guy to think in terms of typical Scheme patterns.

> (define df (make-df* (a 1 2 3 1 2 3) (b 4 5 6 4 5 6) (c 7 8 9 -999 -999 -999)))

> (dataframe-display (dataframe-head df 3))

 dim: 3 rows x 3 cols
       a       b       c 
   <num>   <num>   <num> 
      1.      4.      7. 
      2.      5.      8. 
      3.      6.      9. 

> (dataframe-display df 3)

 dim: 5 rows x 4 cols
     grp     trt   adult     juv 
   <str>   <str>   <num>   <num> 
       a       a      1.     10. 
       a       b      2.     20. 
       b       a      3.     30. 

> (dataframe-display (dataframe-tail df 2))

 dim: 4 rows x 3 cols
       a       b       c 
   <num>   <num>   <num> 
      3.      6.      9. 
      1.      4.   -999. 
      2.      5.   -999. 
      3.      6.   -999.

Read and write

If you are working exclusively with dataframes in Scheme, you can read and write them directly with dataframe-read and dataframe-write. These procedures are straightforward because they are simply reading and writing the dataframe with read and write. 

(define dataframe-write
  (case-lambda
    [(df path) (dataframe-write df path #t)]
    [(df path overwrite)
      (when (and (file-exists? path) (not overwrite))
        (assertion-violation path "file already exists"))
      (when (file-exists? path)
        (delete-file path))
      (with-output-to-file path
        (lambda () (write df)))]))

(define (dataframe-read path)
  (with-input-from-file path read))

Extract values

dataframe-values returns all the values in a column as a list. Following R, I've included $ as an alias for dataframe-values. This procedure is particularly useful when modifying and aggregating dataframes (as I will show in a future blog post).

> (define df (make-df* (a 100 200 300) (b 4 5 6) (c 700 800 900)))

> (dataframe-values df 'b)
(4 5 6)

> ($ df 'b)                 
(4 5 6)

> (map (lambda (name) ($ df name)) '(c a))
((700 800 900) (100 200 300))

> (define df1 (make-df* (x 'b 'a 'b) (y 'd 'e 'c)))

> (remove-duplicates ($ df1 'x))
(b a)

> (remove-duplicates ($ df1 'y))
(d e c)

dataframe-ref returns a dataframe based on a list of row indices and, optionally, the selected column names. I did not follow the principle of least surprise here because dataframe-ref takes a list of indices rather than a single value as in list-ref. For dataframes, the scenario of referencing a single row seemed less likely than a range of rows and I wanted to provide the option to simultaneously select the columns returned. 

> (define df 
    (make-df* 
      (grp "a" "a" "b" "b" "b")
      (trt "a" "b" "a" "b" "b")
      (adult 1 2 3 4 5)
      (juv 10 20 30 40 50)))

> (dataframe-display df)

 dim: 5 rows x 4 cols
     grp     trt   adult     juv 
   <str>   <str>   <num>   <num> 
       a       a      1.     10. 
       a       b      2.     20. 
       b       a      3.     30. 
       b       b      4.     40. 
       b       b      5.     50. 

> (dataframe-display (dataframe-ref df '(0 2 4) 'adult 'juv))

 dim: 3 rows x 2 cols
   adult     juv 
   <num>   <num> 
      1.     10. 
      3.     30. 
      5.     50.